170 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
the forests of Southeast Alaska grow on very steep slopes. The strongly meta 
morphosed rock of the mountains dips at steep angles with the result that land 
slides are frequent and damaging. A knowledge of the location of high hazard 
areas will enable loggers to place their logging operations on more stable lands. 
Again, the location of new roads over difficult terrain can prove very costly in 
construction and maintenance unless potential problem areas van be avoided. 
Careful stereoscopic studies at the time of the road alinements can avoid 
costly errors. Often this requires study of large adjacent areas for evidence 
indicating a lack of stability. 
Discussion 
Dr. R. Smith (Ireland) asked what kind of boundary lines are used. Mr. Retzer answered that 
both taxonomic and combined units are used. 
Mr. R. T. Tomlinson (Canada) wished to hear about the speaker’s experience with helicopters 
in the kind of surveys discussed. Above which elevation does their use become uneconomical 
or impractical? Mr. Retzer answered that helicopters actually have been used. Dr. Francis 
(F.A.O.) added that in the Middle East the Alouette helicopter has proved to be very effective 
to an elevation of about 11,000 ft. 
The speaker gave an affirmative answer to a question by Mr. Mitchell (U.K.) whether 
the 7th approximation was being used for soil classification.